1. Field of the Invention
The present invention is generally related to photolithography systems, and more particularly, to ways of mounting reticles and pellicles.
2. Related Art
In the fabrication of integrated circuits, photolithographic and projection printing techniques are used. In photolithography, an image contained on a reticle is projected onto a wafer having a photosensitive resist thereon. The reticle or mask is used to transfer a desired image onto the silicon wafer. The semiconductor wafer surface is coated with photosensitive resist so that an image is etched thereon. A pellicle may be used in combination with the reticle to protect the reticle surface from damage, and to prevent debris from distorting the image. The pellicle is traditionally mounted on a solid frame to the reticle.
Some wavelengths of light used in photolithography are sensitive to absorption by atmospheric oxygen. Hence, when such oxygen-sensitive light wavelengths are used in photolithography, they must be transmitted through an oxygen-purged atmosphere.
A photolithography system is typically located in a clean room environment. In some situations, the ambient atmosphere of the clean room cannot be purged of oxygen because this may cause other problems with the photolithography process. For instance, a laser interferometer used in a lithography system may be sensitive to changes in the index of refraction of the air, which may occur with a change to an oxygen-free atmosphere. Hence, the oxygen-free environment may have to be restricted to less than the entire lithography system. What is needed is a transmission medium for light wavelengths that have high absorption in an oxygen-containing environment.
A pellicle is generally mounted on a frame opposite a corresponding reticle. Hence, an air gap may exist between the reticle and pellicle. Also, there are no known membrane materials that can be used to protect the patterned reticle surface from particle contamination at the 157 nm exposure wavelength. At present, the preferred alternative is using a thin fused silica window spaced from the reticle by a frame. Typically the frame is bonded both to the reticle and window. Alternately, if both the window and frame are fused silica, and the mating surfaces are sufficiently flat, the reticle, frame and window could be “contacted” together without cement depending on molecular forces to hold the surfaces together. However, the problem of attaching the reticle, pellicle and frame together in an easy manner remains.